Fatty acid synthesis in the rat liver is more effectively inhibited by polyunsaturated fats (PUFA) than saturated fats (SFA). However, chick liver fatty acid synthesis response to PUFA is not appreciably different from that of SFA. The inhibitory effect of fat in the rat is independent of carbohydrate intake but in the chick the depression in lipogenesis associated with dietary fat appears to result from a reduced carbohydrate intake. PUFA exert the maximal inhibitory effect within 4 days, i.e. 50% suppression of 3H2O incorporation into rat liver fatty acids and 50% reduction in the activities of fatty acid synthetase (FAS) and acetyl-CoA carboxylase (AcC). SFA failed to affect these parameters even after 28 days. Total activities of phosphofructokinase and pyruvate kinase were unaffected by PUFA which indicates PUFA inhibition is specific for lipogenic machinery. The decline in fatty acid synthesis following PUFA addition to a fat-free diet precedes any decrease in AcCx or FAS total activities. Preliminary data show that I.V. injected linoleic acid inhibited fatty acid synthesis within 15 min but no effect on assayable AcCx activity; stearic acid injected was much less effective in this respect. The mechanism of PUFA is two-fold - first to block substrate flow to fatty acids and second to specifically induce a suppression in total activity of lipogenic enzymes (e.g., AcCx).